High strength magnesium alloy with excellent flame retardancy, and method for producing same

ABSTRACT

An aspect of the present disclosure relates to a high strength magnesium alloy with excellent flame retardancy, wherein the magnesium alloy comprises 2.0-13.0 wt % of Al, 0.1-0.5 wt % of Mn, 0.0015-0.025 wt % of B, and 0.1-1.0 wt % of Y with the remainder comprising Mg and other unavoidable impurities, and comprises 6.5% or more of an Mg—Al intermetallic compound in terms of volume fraction, the Mg—Al intermetallic compound having an average grain size of 20-500 nm.

TECHNICAL FIELD

The present disclosure relates to a high strength magnesium alloy havingexcellent flame retardancy, and a method of producing the same.

BACKGROUND ART

Magnesium is one of the lightest metals in practical use, and thus, maybe applied to portable electronic products such as smart phones, tabletPCs, laptop computers, and structural materials of transportation meansfor vehicles, trains, aircraft and the like. Magnesium alloys, havingvarious elements added to magnesium, are attracting attention aseco-friendly lightweight metal materials.

Magnesium alloys have excellent castability. Therefore, casting productsof magnesium alloy, which are mainly produced by die casting methodssuch as high pressure casting, low pressure casting or gravity casting,have been mainly applied to practical products. In recent years,development and market expansion of products for use in telegraphmaterials using magnesium alloys, which may be manufactured throughprocessing such as rolling, extrusion or the like, have been promoted.

In general, the types of alloying elements used in magnesium alloys forcasting or magnesium alloys for wrought product are similar. Mostcommonly used types of magnesium alloys are AZ-based alloys with Al andZn added, and AM-based alloys with Al and Mn added. The two types ofalloys commonly contain Al to improve the castability and tensilestrength of magnesium.

The AZ and AM magnesium alloys, which account for most of the commercialmagnesium alloys, are suitable for producing various casting productsdue to improvement in the flowability of molten metal by Al addition,and are also suitable for billet casting and plate casting. However,yield strength or tensile strength thereof is significantly lower thanthose of competitive aluminum alloys, and there is a problem in that athickness of the product should be increased or the product shape shouldbe modified.

In addition, the magnesium alloy has a high possibility of ignition dueto high oxygen affinity, which limits the use conditions.

Therefore, there is a demand for developing a high strength magnesiumalloy having excellent flame retardancy and a method of producing thesame.

PRIOR ART DOCUMENT

(Patent Document 1) Korean Patent Laid-Open Publication No.10-2015-0077494

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide a high strengthmagnesium alloy having excellent flame retardancy and a method ofproducing the same.

On the other hand, the object of the present disclosure is not limitedto the above description. It will be understood by those of ordinaryskill in the art that there is no difficulty in understanding theadditional objects of the present disclosure.

Technical Solution

According to an aspect of the present disclosure, a high strengthmagnesium alloy having excellent flame retardancy includes, by weight %,2.0 to 13.0% of aluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.0015 to0.025% of boron (B), 0.1 to 1.0% of yttrium (Y), a remainder ofmagnesium (Mg), and unavoidable impurities, the high strength magnesiumalloy including a Mg—Al intermetallic compound in a volume fraction of6.5% or more. The Mg—Al intermetallic compound has an average particlediameter of 20 to 500 nm.

According to another aspect of the present disclosure, a method ofproducing a high strength magnesium alloy having excellent flameretardancy includes:

preparing a molten metal including, by weight %, 2.0 to 13.0% ofaluminum (Al), 0.1 to 0.5% of manganese (Mn), 0.0015 to 0.025% of boron(B), 0.1 to 1.0% of yttrium (Y), a remainder of magnesium (Mg), andunavoidable impurities;

casting the molten metal to obtain a magnesium alloy casting material;

subjecting the magnesium alloy casting material to a solution treatmentat a temperature ranging from 370 to 490° C. for 2 to 20 hours to obtaina magnesium alloy;

cooling the magnesium alloy to 100° C. or lower; and

aging the magnesium alloy cooled in the cooling of the magnesium alloyat a temperature of 150 to 250° C. for 2 to 48 hours.

In addition, the solution of the above-mentioned problems does not listall features of the present disclosure. The various features of thepresent disclosure and the advantages and effects thereof may beunderstood in more detail with reference to the following specificembodiments.

Advantageous Effects

According to an embodiment of the present disclosure, there is providedan effect of providing a high strength magnesium alloy having excellentflame retardancy and a method of producing the same.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are images of microstructures of magnesium alloy castingmaterials of comparative material 1(a) and inventive material 7(b).

FIG. 2 is an image of a microstructure after a solution treatment ofcomparative material 1 is completed.

FIG. 3 is an image of a microstructure after a solution treatment ofinventive material 7 is completed.

FIGS. 4A and 4B are graphs illustrating the results of measuringhardness values of comparative material 1(a) and inventive material 7(b)according to the aging time at 200° C.

FIGS. 5A to 5C are images illustrating microstructures of a magnesiumalloy after an aging treatment of comparative member 1(a), inventivematerial 7(b), and comparative material 5 (c).

FIG. 6 is a graph illustrating a change in hardness value based on theaging time of Inventive material 7 and a change in Mg—Al intermetalliccompound size within a crystal grain.

FIG. 7 is a graph illustrating a volume fraction of Mg—Al intermetalliccompound based on the aging time of Inventive Material 7.

BEST MODE FOR INVENTION

Hereinafter, embodiments of the present disclosure will be described.However, the embodiments of the present disclosure may be modified intovarious other forms, and the scope of the present disclosure is notlimited to the embodiments described below. Further, the embodiments ofthe present disclosure are provided to more fully explain the presentdisclosure to those skilled in the art.

As a result of deep research to solve the problems of ignitioncharacteristics and low strength of magnesium alloys, the presentinventors have found that a large amount of intermetallic compounds maybe finely distributed by adding B and Y in combination and performing anaging treatment thereon, and thus excellent flame retardancy and highstrength thereof may be secured, and the present disclosure has beenobtained.

High Strength Magnesium Alloy Having Excellent Flame Retardancy

Hereinafter, a method of producing a high strength magnesium alloyhaving excellent flame retardancy according to an embodiment in thepresent disclosure will be described in detail.

According to an embodiment in the present disclosure, a high strengthmagnesium alloy having excellent flame retardancy includes, by weight %,2.0 to 13.0% of Al, 0.1 to 0.5% of Mn, 0.0015 to 0.025% of B, 0.1 to1.0% of Y, a remainder of Mg, and unavoidable impurities, and includes aMg—Al intermetallic compound in a volume fraction of not less than 6.5%.An average grain size of the Mg—Al intermetallic compound is 20 to 500nm.

First, the alloy composition according to an embodiment will bedescribed in detail. Hereinafter, the unit of each element contentrefers to weight % unless otherwise specified.

Al: 2.0 to 13.0%

Al increases the tensile strength or yield strength, and improves thecastability by improving the fluidity of alloy molten metal.

If the Al content is less than 2.0%, the above-mentioned effect isinsufficient. On the other hand, if the Al content exceeds 13.0%, thebrittleness may be increased and the workability and ductility may bereduced. Therefore, the Al content may be 2.0 to 13.0%.

Further, in detail, a lower limit of the Al content may be 2.5%, and infurther detail, the lower limit thereof may be 6.5% to secure a tensilestrength of 160 MPa or more. In detail, an upper limit of the Al contentmay be 12.0%, and in more detail, the upper limit thereof may be 11.0%.

Mn: 0.1 to 0.5%

Mn is an element contributing to an increase in tensile strength byforming an intermetallic compound with Al to allow for fine grains. Inaddition, Mn serves to lower a corrosion rate of magnesium by loweringFe, which is a typical impurity element unnecessary for a magnesiumalloy, through intermetallic compound formation.

If the Mn content is less than 0.1%, the above-mentioned effect isinsufficient. On the other hand, if the Mn content exceeds 0.5%,brittleness due to excessive formation of an acicular intermetalliccompound may be caused. Therefore, the Mn content may be 0.1 to 0.5%.

Further, in more detail, a lower limit of the Mn content may be 0.11%,and in further detail, an upper limit thereof may be 0.45%.

B: 0.0015 to 0.025%

B (boron) has a significantly high melting point, and solubility thereofin a magnesium solid phase or liquid phase is close to zero. Thus, B isknown as an element not commonly used in general magnesium alloys.

However, in an embodiment of the present disclosure, B is added toensure flame retardancy and high strength. In detail, B contributes toforming a large amount of Mg—Al intermetallic compound by adding B and Yin combination to a magnesium alloy and performing an aging treatmentthereon, thereby improving tensile strength. In addition thereto, inthis case, flame retardancy and strength may be further improved ascompared with the case in which B is added alone. In addition, since Bmay contribute to prevention of oxidation of molten metal to reduce theamount of expensive SF₆ gas used for preventing oxidation of moltenmetal and SO₂ gas which may cause environmental pollution, B maycontribute to reduction in production costs and environmentalprotection.

If the B content is less than 0.0015%, the above-mentioned effect isinsufficient. On the other hand, if the B content exceeds 0.025%, thereis a problem in which an Al—B compound is formed on grain boundaries,reducing ductility. Therefore, the B content may be 0.0015 to 0.025%.

Further, in detail, a lower limit of the B content may be 0.002%, and inmore detail, an upper limit thereof may be 0.02%.

Y: 0.1 to 1.0%

Y bonds with Al to form a precipitate, to contribute to the improvementof strength, and is an element that has a high oxygen affinity to firmlyprotect a surface protective film of molten metal to suppress oxidationof the molten metal. In addition, Y is an element to improve flameretardancy even after solidification.

In addition, as described above, Y is added together with Bincombination to be subjected to an aging treatment, thereby contributingto formation of a large amount of Mg—Al intermetallic compound toimprove tensile strength. Furthermore, in this case, flame retardancymay be further improved as compared with the case in which Y is addedalone.

If the Y content is less than 0.1%, the above-mentioned effect isinsufficient. On the other hand, if the Y content exceeds 1.0%,ductility may be reduced due to formation of a coarse Al—Y compound.Therefore, the Y content may be 0.1 to 1.0%.

Further, in detail, a lower limit of the Y content may be 0.11%, and inmore detail, an upper limit thereof may be 0.95%.

In the embodiment of the present disclosure, the remainder component ismagnesium (Mg). Further, in the ordinary producing process, impuritieswhich are not intended may be mixed from a raw material or a surroundingenvironment, which cannot be excluded. These impurities are known to anyperson skilled in the art, and thus, are not specifically mentioned inthis specification. For example, the impurities may be Fe, Cu, Ni, Ca,Na, Ba, F, S, N or the like.

In this case, in addition to the above alloy composition, 0.3 to 3.0 wt% of Zn may be further included.

Zn: 0.3 to 3.0%

Zn is a solid solution strengthening element and is an element whichpromotes formation of Mg₁₇Al₁₂ phase or improves tensile strength byforming a separate intermetallic compound containing Zn such as in Mg₂Znor the like.

If the Zn content is less than 0.3%, the above-mentioned effect isinsufficient. On the other hand, if the Zn content exceeds 3.0%, a largeamount of a separate intermetallic compound including Zn, such as Mg₂Znor the like, is formed to increase brittleness, which may lead to adecrease in ductility and toughness.

Therefore, the Zn content may be 0.3 to 3.0%. In more detail, the Zncontent may be within a range of from 0.5 to 1.5 wt %, considering theimprovement of the strength and the reduction in brittleness.

A high-strength magnesium alloy having excellent flame retardancyaccording to an embodiment in the present disclosure not only satisfiesthe alloy composition described above, but also contains a Mg—Alintermetallic compound in a volume fraction of 6.5% or more. An averagegrain size of the Mg—Al intermetallic compound is 20 to 500 nm.

When the main alloy element added to magnesium is Al, an Mg—Alintermetallic compound may be formed, and a typical Mg—Al intermetalliccompound is Mg₁₇Al₁₂ phase. The Mg—Al intermetallic compound serves tosecure high strength.

Since a maximum addition amount of Al or other alloying elements to beadded to a magnesium alloy is smaller than a maximum high capacity ofeach alloying element to Mg, most Al is solidified in the Mg matrix,rather than inducing intermetallic compound formation in the grain, andthus, formation of an Mg—Al intermetallic compound is not a generalphenomenon. Thus, it is difficult to form a large amount of an Mg—Alintermetallic compound. In the case of an embodiment in the presentdisclosure, a large amount of Mg—Al intermetallic compound may besecured by adding B and Y in combination and performing an agingtreatment thereon.

If the volume fraction of the Mg—Al intermetallic compound is less than6.5%, it is difficult to ensure high strength. Accordingly, the volumefraction of the Mg—Al intermetallic compound may be 6.5% or more, indetail, 7.0% or more, in further detail, 7.5% or more.

An upper limit of the volume fraction of the Mg—Al intermetalliccompound is not particularly limited, but if the content thereof exceeds30%, the grain size of the Mg—Al intermetallic compound may becoarsened, and brittleness may be increased. Thus, the volume fractionof the Mg—Al intermetallic compound may be 30% or less, and in detail,may be 25% or less.

If the average grain size of the Mg—Al intermetallic compound is lessthan 20 nm, the fraction of the Mg—Al intermetallic compound is low andit is difficult to secure high strength. If the average grain sizethereof is more than 500 nm, brittleness increases.

In this case, one or more of an Al—Mn intermetallic compound and an Al—Yintermetallic compound is further included, and the total amount thereofmay be 5% or less in a volume fraction. If the total amount thereofexceeds 5%, the Mn and Y contents are excessive and thus, brittlenessmay increase.

In this case, the magnesium alloy according to an embodiment in thepresent disclosure may have an ignition temperature of 700° C. orhigher.

In addition, the magnesium alloy according to an embodiment in thepresent disclosure may have a hardness of 70 Hv or more.

The magnesia alloy according to an embodiment in the present disclosuremay have a tensile strength of 130 MPa or more and an elongation of 3%or more. Further, a tensile strength of 160 MPa or more may be securedby controlling the Al content and the like.

Method of Producing High-Strength Magnesium Alloy Having Excellent FlameRetardancy

Hereinafter, a method for producing a high strength magnesium alloyhaving excellent flame retardancy according to another embodiment in thepresent disclosure will be described in detail.

According to another aspect of the present disclosure, there is provideda method of producing a high strength magnesium alloy having excellentflame retardancy, including: preparing a molten metal satisfying theabove-described alloy composition; casting the molten metal to obtain amagnesium alloy casting material; subjecting the magnesium alloy castingmaterial to a solution treatment at a temperature ranging from 370 to490° C. for 2 to 20 hours to obtain a magnesium alloy; cooling themagnesium alloy to 100° C. or lower; and aging the cooled magnesiumalloy at a temperature of 150 to 250° C. for 2 to 48 hours.

Molten Metal Preparation

A molten metal satisfying the above alloy composition is prepared. Themolten metal is prepared through general molten metal preparation formagnesium alloy, without particular limitation.

For example, the above-described alloying elements are prepared inaccordance with the proposed composition range, and then charged into acrucible for melting, to then be subjected to a melting operation. Sincethe melting point of the magnesium alloy is relatively low, any methodusing a gas furnace, an electric furnace, an induction melting furnace,or the like may be used.

In preparing the alloying elements, each alloy element may be preparedin a pure form, but the alloying elements may also be charged into thecrucible in the form of a master alloy in which Mn, B and Y are mixedwith Mg or Al. B, Y and Mn have high melting points and may thus becharged into the crucible in the form of master alloy mixed with Mg orAl, which is advantageous in terms of dissolving.

In addition, when the prepared dissolving material is charged into thecrucible, the material may be charged into the crucible in order fromthe element having a low melting point, which may be advantageous interms of dissolving work.

Casting

The molten metal is cast to obtain a magnesium alloy casting material.The casting is not particularly limited as in the case of the moltenmetal preparation.

For example, a method using a movable mold and a method using a fixedmold may be used. Representative examples of the method using themovable mold include twin roll casting and belt casting using a movablemold such as a twin roll or a twin belt. Also, representative examplesof the method using the fixed mold include semi-continuous casting orcontinuous casting such as billet casting, and may also include moldcasting such as high-pressure casting, low-pressure casting and gravitycasting.

As the casting process, although various methods as described above maybe used, since boron or yttrium having a low solubility with respect tomagnesium is added together with aluminum, it may be advantageous toapply a casting method capable of increasing a cooling rate. To thisend, the mold should be cooled with cooling water. When cooling water isused, the mold surface should be maintained at room temperature or morebefore casting so that the condensed water on the surface of the moldmay be removed, and then should be maintained at room temperature orlower after the condensed water is removed.

Solution Treatment

The magnesium alloy casting material is subjected to a solutiontreatment at a temperature ranging from 370 to 490° C. for 2 to 20 hoursto obtain a magnesium alloy. Since a Mg—Al intermetallic compound isformed in the magnesium alloy casting material, but is formed in theform of coarse Mg—Al or mixed with a Mg matrix (Lamellar Mg—Al), thesolution treatment is performed to enable such an adverse Mg—Alintermetallic compound to a solid solution treatment.

If the solution treatment temperature is less than 370° C. or theholding time is less than 2 hours, the entire Mg—Al intermetalliccompound is difficult to be solidified. If the solution treatmenttemperature exceeds 490° C. or the retention time is more than 20 hours,production costs may be increased and productivity may be lowered, andan ignition phenomenon by oxidation may occur before B and Y are added.Therefore, in more detail, the solution treatment may be carried outwithin a temperature range of 400 to 460° C. for 2 to 20 hours.

Cooling

The magnesium alloy is cooled to 100° C. or lower, to significantlyreduce a natural aging phenomenon that may appear before agingtreatment.

In this case, the cooling rate may be 1 to 100° C./second, and thenatural aging phenomenon that may occur during cooling is significantlyreduced and the solidified Al element may be prevented from beingprecipitated at random. For example, rapid cooling may be preferablyperformed by forced blowing, water cooling, oil cooling, or the like.

Aging Treatment

The cooled magnesium alloy is aged at 150 to 250° C. for 2 to 48 hours.In the microstructure of the cooled magnesium alloy after the solutiontreatment, most of the Al element added as the alloying element does notform a separate intermetallic compound in a state of solid solutionsolidified in the Mg matrix, so that the strength of the material maynot be efficiently increased. Therefore, in the case of an embodiment ofthe present disclosure, a large amount of Mg—Al intermetallic compoundis precipitated through aging to increase the strength and to secureexcellent flame retardancy. For example, when B and Y are combined inthe range suggested according to an embodiment in the presentdisclosure, a large amount of Mg—Al intermetallic compound may beprecipitated through the aging treatment described above.

Since the precipitation by the aging treatment is a solid-phase reactionproceeding in the solid phase, an Mg—Al intermetallic compound having aparticle form, an average particle diameter, a volume fraction, and thelike favorable for the improvement of strength and flame retardancy maybe formed.

If the aging treatment temperature is less than 150° C. or the holdingtime thereof is less than 2 hours, it is difficult to sufficientlysecure the Mg—Al intermetallic compound. On the other hand, if the agingtreatment temperature is more than 250° C. or the retention time is morethan 48 hours, the Mg—Al intermetallic compound may be solidified,resulting in increased production costs and decreased productivity.Therefore, the aging treatment may be performed at 150 to 250° C. for 2to 48 hours. In more detail, the temperature and the holding time may beincreased within the above temperature and holding time depending on theamount of Al added.

MODE FOR INVENTION

Hereinafter, an embodiment in the present disclosure will be describedin more detail by way of example. It should be noted, however, that thefollowing examples are intended to illustrate the present disclosure inmore detail and not to limit the scope of the present disclosure. Thescope of the present disclosure is determined by the matters set forthin the claims and the matters reasonably inferred therefrom.

Embodiment 1

A magnesium alloy casting material having a thickness of 10 mm was castby casting a molten metal having the composition shown in Table 1 below.The magnesium alloy casting material was solution-treated at 420° C. for4 hours, cooled to 20° C., and aged at 200° C. for 12 hours to produce amagnesium alloy.

Mechanical properties of a Mg—Al intermetallic compound of the magnesiumalloy were measured and are shown in Table 1 below. The size of theMg—Al intermetallic compound was measured by an average size obtained bymeasuring a circle-equivalent diameter.

Except for the alloying element shown in Table 1, it was magnesium, andMg—Al means a Mg—Al intermetallic compound.

The ignition temperature was measured at a temperature at which theignition occurred while raising the temperature inside the furnace bodywhile leaving a sample of 10 g in the form of a chip in the furnaceunder the atmosphere.

TABLE 1 Mg—Al Mechanical Properties Ignition Alloy Composition (wt %)Size Fraction TS El Hardness Temperature Classification Al Mn B Y (nm)(vol %) (MPa) (%) (Hv) (° C.) Comparative 9 0.34 — — 193 2.6 113 5.2 59530 Material 1 Comparative 3 0.05 0.013 1.23 178 1.7 113 5.2 48 624Material 2 Comparative 6 0.21 0.001 0.41 205 2.1 121 4.2 53 590 Material3 Comparative 9 0.45 0.412 0.05 232 3.8 124 3.1 63 647 Material 4Comparative 3 0.74 — 0.72 184 1.4 114 4.3 50 634 Material 5 Comparative6 0.12 0.003 1.21 212 2.4 123 3.4 56 660 Material 6 Comparative 9 0.33 —0.21 228 3.1 125 3.2 61 612 Material 7 Inventive 2.8 0.12 0.018 0.92 1347.5 135 5.8 71 721 Material 1 Inventive 2.7 0.32 0.012 0.63 128 8.1 1386.2 72 712 Material 2 Inventive 3.1 0.44 0.006 0.31 133 8.7 142 6.1 74718 Material 3 Inventive 3.4 0.42 0.003 0.13 129 9.8 144 5.7 78 715Material 4 Inventive 5.6 0.12 0.017 0.12 153 13.3 151 4.6 84 747Material 5 Inventive 6.2 0.3  0.012 0.54 161 14.4 155 5.8 87 733Material 6 Inventive 6.2 0.4  0.006 0.81 154 14.7 158 4.7 88 742Material 7 Inventive 5.8 0.42 0.002 0.88 139 13.8 152 4.4 85 739Material 8 Inventive 8.5 0.12 0.013 0.14 176 19.1 164 3.5 103  783Material 9 Inventive 10.6 0.3  0.003 0.51 184 21.6 167 4.6 107  776Material 10 Inventive 9.8 0.4  0.009 0.94 179 20.3 172 4.2 106  782Material 11 Inventive 8.7 0.42 0.019 0.45 172 19.4 168 3.7 104  785Material 12

It can be confirmed that the inventive materials satisfying the alloycomposition and the producing conditions proposed according to anembodiment in the present disclosure include a Mg—Al intermetalliccompound in a volume fraction of 6.5% or more and the average particlediameter of the Mg—Al intermetallic compound satisfies 20 to 500 nm.Also, it can be confirmed that the flame retardancy is excellent as anignition temperature is 700° C. or more, and the mechanical propertiesare also superior to the comparative materials.

Meanwhile, it can be confirmed that the comparative materials satisfiedthe producing conditions proposed according to an embodiment in thepresent disclosure, but did not satisfy the alloy composition, so thatMg—Al intermetallic compounds were not sufficiently secured. Inaddition, it can be confirmed that the flame retardancy is poor and themechanical properties are also inferior to those of the inventivematerials.

Embodiment 2

The changes in the producing process of the comparative material 1 andthe inventive material 7 shown in Table 1 were observed more closely.

FIGS. 1A and 1B are images of the microstructures of the magnesium alloycasting materials of comparative material 1(a) and inventive material7(b). The casting structure of the comparative member 1 is composed of aMg matrix and coarse Mg—Al intermetallic compound, a Mg matrix+Mg—Alintermetallic compound mixed structure (Lamellar Mg—Al), and an Al—Mnintermetallic compound. In the case of the casting structure of theinventive material 7 to which yttrium and boron were added, an Al—Yintermetallic compound (Al—Y) was observed in addition to theabove-mentioned structure, and no boron-containing intermetalliccompound was observed separately.

When the comparative material and the inventive material having theabove casting structure are subjected to the solution treatment, most ofthe Mg—Al intermetallic compounds except for Al—Mn or Al—Y intermetalliccompounds are solidified as the base structure as illustrated in FIGS. 2and 3, and thus, is not observed. The optical structures of the castingstructure and the solution treatment material are almost similar exceptfor the presence or absence of the Al—Y intermetallic compound, but showa large difference in the aging treatment material.

These differences may be confirmed first in the hardness measurementresults according to aging time. As illustrated in FIGS. 4A and 4B, thehardness values of the comparative material 1(a) and the inventivematerial 7(b) were measured at 200° C. according to the aging time. As aresult, it can be confirmed that the hardness of the inventive materialis significantly high. Also, the hardness value of the comparativematerial 1 barely changes according to the aging time, but it can beconfirmed that the hardness value of the inventive material greatlyincreases when the aging time exceeds 1 hour. In detail, after the lapseof 3 hours, a maximum hardness value corresponding to peak aging isshown, and the hardness value at this time is a value of 97 to 107 Hvwhich is a value increased 60% or more, as compared with the averagehardness value of the casting material aged for 1 hour or less. Inaddition, a maximum hardness value of the inventive material 7 is abouttwo times the maximum hardness value of the comparative material.

FIGS. 5A to 5C are images showing the microstructures of the magnesiumalloys after the aging treatment of the comparative material 1(a), theinventive material 7(b) and the comparative material 5(c). It can beconfirmed that a large amount of Mg—Al intermetallic compound having asize of several tens of nanometers is precipitated in the inventivematerial 7, which shows that the hardness value of the inventivematerial is significantly increased.

FIG. 6 illustrates changes in the hardness value (rhombus) and the size(square) of the Mg—Al intermetallic compound in the crystal grains withrespect to the aging time of the inventive material 7, and FIG. 7illustrates a volume fraction of Mg—Al intermetallic compounds withaging time. As illustrated in FIGS. 6 and 7, when the inventive material7 is aged for 3 hours or more, it can be seen that the Mg—Alintermetallic compounds are grown to 20 nm or more and 10 vol % or more,respectively, in the average size and the volume fraction.

While embodiments have been illustrated and described above, it will beapparent to those skilled in the art that modifications and variationscould be made without departing from the scope of the present disclosureas defined by the appended claims.

1. A high strength magnesium alloy having excellent flame retardancy,comprising: by weight %, 2.0 to 13.0% of aluminum (Al), 0.1 to 0.5% ofmanganese (Mn), 0.0015 to 0.025% of boron (B), 0.1 to 1.0% of yttrium(Y), a remainder of magnesium (Mg), and unavoidable impurities, themagnesium alloy comprising a Mg—Al intermetallic compound in a volumefraction of 6.5% or more, wherein the Mg—Al intermetallic compound hasan average particle diameter of 20 to 500 nm.
 2. The high strengthmagnesium alloy having excellent flame retardancy of claim 1, whereinthe magnesium alloy further comprises 0.5 to 1.5 weight % of zinc (Zn).3. The high strength magnesium alloy having excellent flame retardancyof claim 1, wherein the magnesium alloy further comprises one or more ofan Al—Mn intermetallic compound and an Al—Y intermetallic compound, anda total content of the Al—Mn intermetallic compound and the Al—Yintermetallic compound is 5% or less in a volume fraction.
 4. The highstrength magnesium alloy having excellent flame retardancy of claim 1,wherein the magnesium alloy has an ignition temperature of 700° C. orhigher.
 5. The high strength magnesium alloy having excellent flameretardancy of claim 1, wherein the magnesium alloy is a hardness of 70Hv or more.
 6. The high strength magnesium alloy having excellent flameretardancy of claim 1, wherein the magnesium alloy has a tensilestrength of 130 MPa or more and an elongation of 3% or more.
 7. A methodof producing a high strength magnesium alloy having excellent flameretardancy, the method comprising: preparing a molten metal including,by weight %, 2.0 to 13.0% of aluminum (Al), 0.1 to 0.5% of manganese(Mn), 0.0015 to 0.025% of boron (B), 0.1 to 1.0% of yttrium (Y), aremainder of magnesium (Mg), and unavoidable impurities; casting themolten metal to obtain a magnesium alloy casting material; subjectingthe magnesium alloy casting material to a solution treatment at atemperature ranging from 370 to 490° C. for 2 to 20 hours to obtain amagnesium alloy; cooling the magnesium alloy to 100° C. or lower; andaging the magnesium alloy cooled in the cooling of the magnesium alloyat a temperature of 150 to 250° C. for 2 to 48 hours.
 8. The method ofproducing a high strength magnesium alloy having excellent flameretardancy of claim 7, wherein the molten metal further comprises, byweight %, 0.5 to 1.5% of Zn.
 9. The method of producing a high strengthmagnesium alloy having excellent flame retardancy of claim 7, whereinthe preparing of the molten metal is performed by charging a cruciblewith Mn, B and Y mixed with Mg or Al in the form of a master alloy. 10.The method of producing a high strength magnesium alloy having excellentflame retardancy of claim 7, wherein the preparing of the molten metalis performed by charging the crucible sequentially from an elementhaving a low melting point.
 11. The method of producing a high strengthmagnesium alloy having excellent flame retardancy of claim 7, whereinthe cooling is performed at a cooling rate of 1 to 100° C./second.